1. Field of the Invention
This invention relates to automotive suspension systems, and more particularly to a method and apparatus for absorbing mechanical shock.
2. Description of Related Art
Shock absorbers are used in conjunction with automotive suspension systems to absorb unwanted vibration which occur during driving. To absorb this unwanted vibration, shock absorbers are generally connected between the body and the suspension of the automobile. A piston is located within the shock absorber and is connected to the body of the automobile through a piston rod. Because the piston is able to limit the flow of damping fluid within the working chamber of the shock absorber when the shock absorber is compressed, the shock absorber is able to produce a damping force which counteracts the vibration which would otherwise be transmitted from the suspension to the body. The greater the degree to which the flow of damping fluid within the working chamber is restricted by the piston, the greater the damping forces which are generated by the shock absorber.
In selecting the amount of damping that a shock absorber is to provide, three vehicle performance characteristics are often considered: ride comfort, vehicle handling and road holding ability. Ride comfort is often a function of the spring constant of the main springs of the vehicle, as well as the spring constant of the seat, tires, and the shock absorber. Vehicle handling is related to the variation in the vehicle's attitude (i.e., roll, pitch and yaw). For optimum vehicle handling, relatively large damping forces are required to avoid excessively rapid variation in the vehicle's attitude during cornering, acceleration, and deceleration. Road holding ability is generally a function of the amount of contact between the tires and the ground. To optimize road holding ability, large damping forces are required when driving on irregular surfaces to prevent loss of contact between the wheels and the ground for an excessive period of time.
To optimize ride comfort, vehicle handling, and road holding ability, it is generally desirable to have the damping forces generated by the shock absorber be responsive to the input frequency from the road. When the input frequency from the road is approximately equal to the natural frequency of the body of the automobile (e.g., approximately between 0-2 Hz), it is generally desirable to have the shock absorber provide large damping forces to avoid excessively rapid variation the vehicle's attitude during cornering, acceleration and deceleration. When the input frequency from the road is between 2-10 Hz, it is generally desirable to have the shock absorber provide low damping forces so as to produce a smooth ride and allow the wheels to follow changes in road elevation. When the input frequency from the road is approximately equal to the natural frequency of the automobile suspension (i.e., approximately 10-15 Hz), it may be desirable to have relatively low damping forces to provide a smooth ride, while providing sufficiently high damping forces so as to prevent excessive loss of contact between the wheels and the ground.
One method for selectively changing the damping characteristics of a shock absorber is disclosed in U.S. Pat. No. 4,597,411. In t his reference, a solenoid is used to selectively open and close an auxiliary opening in a base valve of a shock absorber. The base valve then regulates the pressure inside one portion of the working chamber of the shock absorber so as to control damping. Another method for selectively changing the damping characteristics of a shock absorber is disclosed in PCT/SE 86/00212. In one embodiment, this reference discloses the use of a pressure sensor to count the number of compression-rebound cycles of the absorber, as well as an accelerometer attached to the wheel support to determine the vertical velocity of the body of the automobile. The damping characteristics of the absorber are then changed in response to the vertical velocity of the body.
A further method for selectively changing damping characteristics of shock absorbers is disclosed in United Kingdom Patent Application GB 2 147 683 A. In one embodiment, this reference discloses a valve disk which is used to cover channels in a valve body which transfers damping fluid between the upper and lower portions of the working chamber. The valve disk is biased against the valve body by a support member which is disposed partially within a pressure chamber. The pressure chamber communicates with the lower portion of the working chamber through a first flow path, and to the upper portion of the working chamber through a second flow path. To regulate the flow of damping fluid through the second flow path and hence the pressure in the pressure chamber acting on the support member, an auxiliary valve plate is provided. The auxiliary valve plate is disposed over the second flow path and cooperates with a coil which is located on the valve body below a portion of the auxiliary valve plate. When the coil is energized, the magnetic flux generated by the coil produces a biasing force on the auxiliary valve disk causing the auxiliary valve disk to deflect, thereby increasing the opening between the second flow path and the upper portion of the working chamber. Accordingly, when the coil biases the auxiliary valve disk in the position to allow more hydraulic fluid to flow through the second flow path, the pressure of the damping fluid in the pressure chamber declines thereby reducing the force transmitted to the valve plate by the support member. The pressure in the lower portion of the working chamber causes the valve plate to deflect, thereby increasing the amount of damping fluid flowing through the channels.